A radio access network of a wireless or cellular communication network comprises a plurality of radio base stations, RBSs, distributed over an area. The area may be a region, a city, a country or several countries. Generally, each RBS is associated with a coverage area which is commonly referred to as a cell.
In a wireless or cellular communication network, users having user equipments, UEs, may move around causing the traffic load in each cell or RBS to vary over time. As a result, some RBSs may experience very heavy traffic loads at certain times.
The geography of a wireless or cellular communication network may vary from cell to cell and also within a cell. For example, in a city there may be buildings of different heights and sizes, there may be roads or streets of different sizes and constitutions from cell to cell and also within a single cell.
Due to the variations in traffic loads over time, there may be certain areas, e.g. within a cell, which suffer from either a traffic load exceeding the capacity of the RBS of that cell, e.g. due to a large number of users at these certain areas. Due to the variations in geography, there may be certain areas, e.g. within a cell, which suffer from poor coverage, e.g. due to radio shadow caused by a building or the like.
One way to cope with these problems and to be able to provide services to users to the largest extent possible, low power RBSs are employed. A low power RBS is a RBS which has substantially lower transmit power than a regular RBS. A regular RBS is also referred to as a macro RBS. A low power RBS has a much smaller coverage area, or cell, than a macro RBS due to its reduced transmit power. The cell of a macro RBS is also referred to as a macro cell and the cell of a low power RBS is also referred to as a low power cell. A low power RBS are also referred to as a micro, pico, femto RBS depending on its transmit power. The plurality of macro RBSs and the low power RBSs may have whole or partly overlapping coverage areas. Often, a low power RBS may be placed within the coverage area of a macro RBS. The deployment of macro RBSs and low power RBSs are also called Heterogeneous network deployment or HetNet.
The HetNet deployment may also be used to handle a large traffic growth wherein low power RBS are added to increase capacity of the radio access network of the wireless or cellular communication network. The HetNet deployment may also be used to extend network coverage to areas with no macro coverage. The output power from the low power RBSs, having small cells or coverage areas, is typically several times smaller compared to the macro RBSs and this difference creates an imbalance between the uplink and downlink. A network with a large difference in output power among the cells will have different optimum cell borders for uplink and downlink as indicated in FIG. 1a. FIG. 1a is a schematic illustration of a macro radio base station and a low power radio base station.
From FIG. 1a, it is shown that a macro user, i.e. a UE currently being served by the macro RBS but located at the cell edge or border between the macro and the low power RBS will have a lower pathloss to the low power RBS and may therefore cause a lot of interference to the low power RBS. One way to handle the caused interference is to increase uplink power control target for UE(s) connected to the low power RBS. In order to fully compensate for the difference in downlink output power, the uplink power control target for the low power RBS should be increased with the same amount.
When low power nodes or RBSs are deployed in an existing macro layer using the same frequency, the downlink power imbalance could create problems on the uplink. A macro user, i.e. a UE currently being served by a macro RBS on the cell edge to the low power RBS will adjust its output power to reach a certain signal strength level in the macro cell but due to the downlink imbalance in output power the macro user will have a lower pathloss to the low power RBS and will therefore create a very high interference level in the low power RBS. This is illustrated in FIG. 1b and one straight forward solution to this is to increase the power control target in the low power RBS to match the difference in downlink output power, if the difference in downlink output power between the macro RBS and the small RBS is 13 dB then the uplink power control target in the small cell should be adjusted with 13 dB. So if the default uplink power control target for the macro layer is −103 dBm the uplink power control target for the pico layer should be set to −90 dBm. However, the use of this more aggressive uplink power control target in the small cell may create more interference and may also result in increased drain of batteries in the UEs.
                                          SINR                          UE              1                                =                                                                      P                  1                                *                                  G                                      1                    1                                                                                                                    P                    2                                    *                                      G                                          2                      1                                                                      +                                  N                  0                                                      =                                          -                                  1                  3                                            ⁢              dB                                      ,                            (        1        )            where P1 is the uplink transmission power for UE1 connected to the low power RBS 102 and G11 is the gain from UE1 to the low power RBS 102. Further, P2 is the uplink transmission power for UE2 connected to the macro RBS 101 and G21 is the gain from UE2 to the low power RBS 102.